The title of the post says it all. The handy “?rel=0” code add-on at the end of the embedded URL no longer works. It has been disabled by YouTube.
This handy code snippet stopped unrelated videos appearing after the embedded video was played. It was ideal for school websites which had to be very careful about the content they displayed.
Obviously overuse of the so-called “trick” was interfering with YouTube’s revenue streams. What a pity?